The preparation of polycarbonate can be achieved through the melt reaction of an aromatic dihydroxy compound and a diaryl carbonate. There are several methods by which diaryl carbonate can be produced including decarbonylating a diaryl oxalate in the presence of a catalyst while removing a carbon monoxide by product; reacting an aromatic hydroxy compound with phosgene in the gas phase in the presence of a heterogeneous catalyst, for example, the direct phosgenation of phenol; reacting an aromatic hydroxy compound, carbon monoxide, and oxygen in the presence of a redox catalyst and an organic salt; or reacting an aromatic hydroxy compound with a dialkyl carbonate. A specific example of a non-phosgene route to synthesize the diaryl carbonate of diphenyl carbonate (DPC) can be achieved with the use of respective catalysts through the transesterification of dimethyl carbonate (DMC) and phenol to produce phenyl methyl carbonate (PMC) as shown in Reaction (1),
followed by the subsequent disproportionation of PMC to produce diphenyl carbonate (DPC) as shown in Reaction (2),
with an additional formation of small amounts of an alkyl aryl ether (anisole) as the main reaction byproduct.
The formation of diaryl carbonates in any of the aforementioned reaction schemes or in any other reaction scheme can generally be facilitated through the use of a catalyst. Unfortunately, any residual metal from said catalyst can result in discoloration of the resultant polycarbonate and a reduction in the color stability of the polycarbonate. Furthermore, the metal from the catalyst used in the formation of the diaryl carbonate can cause corrosion of the process vessels that can result in a further source of metal corrosion, in addition to any degradation of the process vessels that can occur independently of the catalyst.
Current processes to purify diphenyl carbonate from the catalyst involve the use of a set of separating columns, in which the transesterification reaction mixture is introduced to a high boiling point material separating column where a purified diphenyl carbonate is produced as a top component and diphenyl carbonate containing the catalyst exits as a bottom component. However, an amount of residual catalyst is generally still present after said purification, due to their relative volatility or entrainment. This remaining amount of catalyst can result in discoloration when used as a reactant in the polymerization of polycarbonate.
An improved method to purify the diaryl carbonate reactant from metal contaminants that arise from either or both of the catalyst or the degradation of the process vessels would therefore be desirable in the production of polycarbonate for use in good color stability and high transparency applications.